Designing Robust Constructors (and Perhaps Destructors)

When designing a class, one of the most important design issues involves how the class will be constructed . Constructors are discussed in Chapter 3, "Advanced Object-Oriented Concepts." Revisit this discussion if you need a refresher on guidelines for designing constructors.

First and foremost, a constructor should put an object into a safe state. This includes issues such as attribute initialization and memory management. Scott Meyers discusses some of these issues in the section "Constructors, Destructors and Assignment Operators" of Effective C++ . You also need to make sure the object is constructed properly in the default condition. It is normally a good idea to provide a constructor to handle this default situation.

In languages that include destructors, it is of vital importance that the destructors include proper clean-up functions. In most cases, this clean-up pertains to releasing system memory that the object acquired at some point. Java and C# reclaim memory automatically with a garbage collection mechanism. In languages such as C++, the developer must include code in the destructor to properly free up the memory that the object acquired during its existence. If this function is ignored, a memory leak will result.

Memory Leaks

When an object fails to properly release the memory that it acquired during an object's life cycle, the memory is lost to the entire operating system as long as the application that created the object is executing. For example, suppose multiple objects of the same class are created and then destroyed , perhaps in some sort of loop. If these objects fail to release their memory when they go out of scope, this memory leak slowly depletes the available pool of system memory. At some point, it is possible that enough memory will be consumed that the system will have no available memory left to allocate. This means that any application executing in the system would be unable to acquire any memory. This could put the application in an unsafe state and even lock up the system.